The real reason was of course it was silly to have a new guest engineer every week and they needed to shift one of the regulars down there. Apparently that was a big oversight when they originally cast everyone.

For an in universe explanation Picard said he picked Geordi for his ship when he was impressed that Geordi put in some hard work fixing a shuttle when Picard mentioned it wasn't working at peak efficiency. So Picard chose him as his navigator, and had him on his radar to advance once he had served for a little while. Apparently that work ethic impressed Picard a lot. By Geordi's own admission he was an average pilot at best.